An interface between a GC and MS is usually required since the carrier gas flow rate through a GC is relatively high and emerges from the GC at atmospheric pressure; while the MS must operate under high vacuum conditions and as a result has a relatively low intake capacity for carrier gas. An additional important requirement is that air from the atmosphere must be excluded from the entire system, especially the MS, in order to ensure good instrument performance. The main objective is to combine the sample separating capabilities of the GC with the sample identification capabilites of the MS into one continuous analytical operation with a minimum decrease in the performance levels of the combined instruments operating separately. This objective has not been fully realized. In general, quantitative analysis is not as sensitive nor as reliable with combined GC/MS analysis as with stand alone GC analysis. The reproducibility, accuracy and level of detection for the analysis of trace components or constituents of a given sample mixture has been sacrificed somewhat in order to be able to identify each component by the use of the MS.
The reason for this deterioration in performance, aside from some differences in the method of measurement, is that the effects of any GC/MS interface on the relative amounts of sample or carrier gas which passes through it are not well defined and are sensitive to slight variations in the experimental pressure/flow conditions for a given analysis.
Interface performance may be conveniently characterized by three definitions: (1) the percent transmission of sample through the interface or the "percent yield;" (2) the percent transmission of carrier gas or the "split;" and (3) the ratio of (1) to (2) or the sample "enrichment factor." An obvious objective for any analysis is a 100% yield. A low split may be compensated for by a high enrichment factor. Also, the GC may be required to operate under low flow conditions in which the constituents of the sample are not separated from each other as well as they might be for stand alone GC analysis.
There are sevral methods currently used to interface a GC to an MS. One solution is the "direct connect" method. There is no actual interface device. Instead, the interface is eliminated and the output end of the GC column is inserted directly into the MS. The "direct connect" method when applicable, shares with the present inventions the capability of reducing the pressure at the end of the GC column to such a low level that a simplified pressure/flow relationship applies and the GC flow rate becomes independent of the pressure at the end of the GC column. This is important for reproducible quantitative analysis as discussed previously. However, this poses several problems. Most important, the flow rate of the sample through the GC column is usually too high for the MS to handle. This requires the GC to operate at a lower than optimum flow rate, which results in broader peaks on the strip charts produced by the GC. In addition, this type of connection makes it inconvenient to use either the GC or MS alone.
One further shortcoming of the direct method is that solvents in which the sample mixture is dissolved and also other major sample components may interfere with the MS analysis of the remaining sample components. While various types of interfaces can divert these objectionable sample components from the MS without disturbing the continuous GC/MS analysis (usually via a diversion valve), this is not possible with the "direct connect" method.
Another method of interfacing a GC to an MS is via an open split interface. The interface is essentially an enclosed chamber into which both the GC and MS columns are inserted into opposite ends along the same axis. An important feature of this interface is the very narrow bore of the MS transfer capillary, the impedance of which determines the fixed flow rate to the MS, provided atmospheric pressure is maintained in the interface. The interface also contains input and output make-up gas columns (vents). The excess flow from the GC exits through the output column. If the GC flow rate happens to be too low, make-up carrier gas must be added through the input column in order to maintain a constant MS flow rate and, more importantly, to exclude air from leaking in through the output vent and into the MS.
Several problems, however, remain with the "open split" interface. The MS transfer column is generally an uncoated glass surface which effects the flow of sample in a different manner than the surface of the GC column. This effect is in addition to chemical interaction with highly reactive sample compounds (which is minimized by the use of relatively inert fused silica glass). The physical adsorption of the sample on the glass surface, unlike the liquid film surface of the GC column will effect the flow of the sample in a non-uniform and unpredictable manner. Condensation on the glass surface can also cause broad, skewed (non-symmetrical) sample peaks as detected by the MS. This effect becomes particularly important for very large molecular weight compounds which have a far greater tendency to condense at the maximum temperature of the interface. The inability to analyse chemically important large molecules is a critical restriction on GC/MS applications.
Further problems with the open split interface make it difficult to stabilize interface parameters, thus effecting the reproducibility of an analysis. If the GC flow is greater than the MS flow, the flow is split (the excess flow exits through the output vent), which results in sample loss. Although sample loss alone is not a critical problem in most instances, the open split interface does not provide a means for controlling the flow split. The flow split is dependent primarily on the GC flow rate. The MS flow rate is dependent on maintaining a fixed pressure in the interface. This requires that adjustments be made in make-up (input) gas flow and output vent gas flow for different GC flow rates.
In addition, if air is to be effectively purged from the interface by the output flow of the make-up gas, then the output vent must be relatively small or, alternatively, the flow must be relatively high, either of which increases the interface pressure to well above atmospheric levels. Since both the MS and GC gas flows are dependent on the square of the pressure in the interface (the GS flow goes down and the MS flow goes up with increasing pressure), the net result is an unstable condition which must be "tuned" by trial and error for each GC analysis. The problem is further complicated by the fact that the GC flow normally decreases by a gradual but significant amount during the course of analysis because of an increase in the viscosity of a gas as its temperature increases. Thus, interface parameters may require adjustment depending on the conditions for the GC analysis and as a result of normal changes of GC flow rate during the analysis. Altogether, there is a high probability of having significant differences in GC/MS flow parameters from one analysis to another.
A jet separator, such as that disclosed in Bradley, U.S. Pat. No. 3,936,374, can also be used to interface a GC to an MS. The jet separator is similar to the present invention except that a jet separator requires a short restrictor capillary or "jet" to be used at the end of the GC column in order to produce a supersonic jet in the interface between this restrictor and an almost equally narrow MS column across a very narrow gap. As a combined result of this very high linear velocity and the very high diffusion rate of the low molecular weight carrier gas into the interface chamber, a greatly reduced flow of carrier gas which has been "enriched" in the relative concentration of sample enters the MS column.
This has been the traditional GC/MS interface for high GC flow analysis, where the GC flow rate far exceeds the capacity of the MS. More recently with the development of much smaller capillary GC columns, high flow rate GC analysis is no longer required, though typically the flow rate is still too high for most MS instruments. However, the enrichment mechanism does not function at these relatively low flow rates. This problem may be solved by the addition of make up gas just in front of the interface in order to restore the enrichment mechanism to the extent that the net result may be a sample yield of roughly 40 to 60 percent depending on the sample molecular weight, with an optimized GC flow rate and an MS pressure at an acceptable level. The enrichment factor may then be at least approximately independent of the GC flow rate.
The primary problem with the jet separator interface is again, the active surface of the interface, which may have unpredictable interactions with reactive sample components and cause condensation of very high molecular compounds which results in skewed or asymmetric peaks. Also, isolation of the two instruments is not easily accomplished although there is usually solvent diverting capability. Nor is there a reliable or accurate mechanism to adjust carrier gas splitting in the interface.
Another significant problem with a jet separator interface, as with the "open split" interface, is that the relatively high exit pressure of the GC column still affects the GC flow rate and leads to unstable and difficult to reproduce pressure/flow conditions in the interface. The reliability of quantitative capillary GC/MS analysis with a jet separator, as with the "open split" interface, has not been clearly demonstrated.